The division of eukaryotic cells must occur in a highly faithful manner. During the course of the cell cycle, cells must be able to replicate their DNA with great fidelity. Furthermore, when problems arise during replication, cells must impose a checkpoint-mediated arrest of the cell cycle while they attempt to rectify lesions. Recently, we identified a novel protein called Treslin that is essential for DNA replication in vertebrate cells Treslin acts at a critical regulatory juncture in cellular duplication. In particular, Treslin is akey target of the cyclin-dependent kinase (CDK) that promotes the initial firing of replication origins at the onset of S-phase. Moreover, Treslin also appears to participate in checkpoint regulation. For the studies in this proposal, we will carry out an intensive analysis of Treslin and its functional relationships with other key regulators of S-phase. We will conduct these investigations in both Xenopus egg extracts and human cells. The egg-extract system offers some technical advantages that are not available currently with human cells. Numerous studies have indicated that this Xenopus system offers a valid model for human cells. Overall, we will attempt to reveal how Treslin promotes accurate replication and maintenance of the genome in vertebrates and how cells control the activity of Treslin. In particular, we will analyze the varios domains of Treslin in order to elucidate its functional architecture. We will examine how the collaboration of Treslin with other replication proteins contributes to its function. We will study further how phosphorylation controls the functional properties of Treslin. Moreover, we will also investigate the role of Treslin in checkpoint responses. Finally, we will attempt to identify new partners of Treslin and elucidate potentially novel functions of this protein. In general, these studies hold the promise to yield valuable insights into how cells maintain genomic integrity throughout their lifetimes. This information would be especially relevant for human health. Derangement of genomic integrity as a consequence of environmental insults or inherited mutations or both can result in various human pathologies, most notably cancer. Thus, a thorough understanding of the root causes for genomic instability will be essential for an informed strategy in combating cancer.